Many different methods have been developed for the production of board products by reducing wood or other lignocellulose products to fine particles and forming such particles into a unitary mat, followed by consolidating the mat into a solid board by the application of heat and pressure.
In general, two of such methods are the wet process and the dry process. In the wet process a pulp mat is prepared from aqueous slurry of wood fibers and formed into a thick sheet in a manner similar to the conventional paper manufacturing process and the mat is pressed into a fiber board panel. In the dry process an aqueous slurry is never used. The fibers have a moisture content of from about 5% to 100% based on the dry weight of the fiber. The term "dry process" or "dry formed" usually indicates that the fibers are conveyed in a gaseous stream rather than a liquid stream to a filter and formed into a mat which is consolidated while still moist into hardboard by the application of heat and pressure. However, particles or chip board materials may be mechanically conveyed in the dry process for felting or forming. In the manufacture of dry process hardboard using phenolic or modified phenolic resins, curing of the resin and possibly stabilizing the panel is accomplished by pressing between platens heated to 400.degree. F. or above. Even though the process is termed "dry," moisture generally in the range of 5 to 8% is required for good consolidation. It has been found that in thick panels, particularly those above 1/4 inches in thickness, it is difficult to secure surfaces that are not soft. These surfaces have low abrasion resistance, and are absorptive of paint or adhesive material. Generally, soft surfaces on hardboard and particleboard have been attributed to pre-cure of the resins.
However, it is the inventor's contention that it is the migration of moisture away from the surface before the resin flows to bond the fibers that result in soft surfaces. It has been found that panels are improved by the addition of moisture to mat surfaces prior to their entering the press. To prevent the rapid migration of the moisture away from the surface of the panel before the resin flows to bond the fibers, the use of surface added moisturizers for hardboard manufacture has been employed, and this technique is the subject matter of applicant's co-pending application, U.S. Ser. No. 289,370, now abandoned file Aug. 31st, 1972, and entitled "Surface Added Plasterizers For Hardboard Manufacture." In applicant's co-pending application, a mixture of water and ethylene glycol as a surface spray is employed in the manufacture of panels resulting in a superior surface. Surface flow or consolidation is obtained. However, an undesirable side effect is an increase in the tendency of the panel to stick to the plates against which they are pressed during the manufacturing process. To overcome this, the process may require the steps of subsequently sanding the manufactured panels.
The shortcoming of sticking of the panel to the platen plates is not acceptable for panels that are to be finished without sanding. Thus, in applications wherein the finished product is a 7/16 inches siding having a smooth or textured surface, finishing by sanding is not acceptable. Smooth panels are primed for finishing painting in the field, or overlaid with a polyfluoride film. On the other hand, textured panels are sold raw, primed, or finished.
Thus, it is an object of the subject invention to eliminate sticking of the resulting hardboard panel, and this is achieved by the application of a dry paper overlay, such as newsprint which may be most easily obtained, on the hardboard panel prior to final curing. It has been found that a good bond is achieved between the paper overlay and the panel by the use of a mixture of phenolic resin and the preferred plastisizer, ethylene glycol. It is a combination of these components that provide the satisfactory bond, with enough of the resin impregnating the paper to result in a composite panel having excellent resistance to weathering. It has also been found that overlaid panels, made according to the subject invention, have excellent finishing properties, have good paint holdout, resistance to scuffing, and are easily and cleanly cut.
During the manufacture of the subject composite panel, use is made of a polyhydric compound such as a polyhydric alcohol, an ester thereof, or a low polymer of ethylene glycol, having a boiling point above that of water so that it does not move readily from the surface of the panel during heating, and makes it possible to obtain a superior panel when the alcohol or ester is used in conjunction with a phenolic or similar resin to bond the paper overlay to hardboard during pressing. Two polyhydric compounds which will satisfy the requirements of having a boiling point above that of water are ethylene glycol and glycerine. The esters of these compounds have intermediate boiling point ranges, and have one or two hydroxy radicals, and may be employed for materials (such as particleboard or flake board) which are cured at lower temperatures.
In summary, the subject invention eliminates the disadvantages of conventional hardboard and particleboard made by the dry process, and specially in board thicker than 1/4 inches, wherein the surfaces of such boards have low abrasion resistance, and are very absorptive of paint or adhesives. The subject invention eliminates these disadvantages by forming a hard, unitary, waterproof coating on the top surface of the hardboard. This coating includes a layer of dry paper, e.g., newsprint, added to a hardboard. The surface of the latter is wet with a solution of ethylene glycol and phenolic resin. The composite structure is cured by the application of heat and pressure. Composite panels produced in this manner are more stable and can be painted. They can be removed from the platens in the press without sticking. It is understood that the mass of the defibrated wood chips are bound together by hemicelluloses, lignins, and other natural binding elements of the wood. If desired, additional ingredients can be added, such as glues, waxes, and other binding materials. One of the particular features of the invention is the provision of an overlay ply of dry paper. The dry paper ply is added to the hardboard mat before curing and produces a nonsticking surface.
The foregoing and other objects and advantages of the present invention will be more readily apparent from the inspection of the accompanying drawings, taken in connection with the following specifications describing in detail an embodiment of the products produced and the method of making same according to the subject invention.